Cosmarium scutiforme Van Geest et Coesel, 2019

Geest, Alfred Van & Coesel, Peter, 2019, Some new and interesting desmids (Streptophyta, Desmidiales) from ephemeral puddles in the urban and industrial areas of Amsterdam (Netherlands), Phytotaxa 387 (2), pp. 119-128 : 126

Cosmarium scutiforme Van Geest et Coesel sp. nov. (Figs. 7, 18, 24)

Diagnosis: Cells in frontal view elliptic, moderately constricted, about 1.5 times longer than broad, with a shallow, almost closed sinus. Semicells pyramidal with a broadly rounded apex and convex sides. Sides crenate with about 18 crenations that slightly increase in width from the sinus to the apex. On the inner side of the margin there are some two additional series of flattened verrucae visible. The centre of the semicell is distinctly punctate. Lateral view elliptic with a shallow obtusely angled constriction. Apical view elliptic-rhombic. Chloroplast with a single, central pyrenoid. Dimensions: cell length 38–41μm, cell breadth 26–27μm, cell thickness 19–20μm, breadth of isthmus 15–17 μm.

Type:— THE NETHERLANDS. Amsterdam: rain puddle, 52.33838 N, 4.91980 E, Van Geest , 18 Nov 2015 (holotype L! Nationaal Herbarium Nederland, Hugo de Vries Lab. no 2018.09, preserved as a fixed natural sample).

Etymology: Latin ‘scutu’ means ‘shield’, so ‘scutiforme’ means ‘shield shaped’.

At first glance, the taxon under discussion reminded us of Cosmarium microsphinctum var. crispulum Nordstedt ( Nordstedt & Wittrock 1876) albeit it that the cell apex in our specimens is broader rounded than that in Nordstedt’s (1875, pl. 6: 5) illustration. However, when observing empty cells, some faintly developed series of submarginal verrucae could be distinguished where C. microsphinctum Nordstedt (1876) is characterized by smooth-walled cells. In respect of that, there is more similarity with C. speciosum var. simplex Nordstedt (1872: 31 , pl. 6: 12). Main difference is in the outline of the semicell: semi-elliptic in C. speciosum var. simplex , pyramidal in our alga under discussion. Later on, however, various formae of C. speciosum var. simplex have been described varying in cell size, cell outline and number of crenations, see Růžička (1957).

Our alga could be rendered a place in that series of formae but we are of opinion that it deserves the status of a separate species as it differs in too many aspects with C. speciosum as originally described by Lundell (1871). C. scutiforme was found at location 4 and 5.